1. Field of the Invention
This invention relates to process control systems. More specifically, the present invention relates to a process control system which automatically senses connection of process devices and automatically configures the devices when sensed.
2. Description of the Related Art
Present-day process control systems use instruments, control devices and communication systems to monitor and manipulate control elements, such as valves and switches, to maintain at selected target values one or more process variables, including temperature, pressure, flow and the Eke. The process variables are selected and controlled to achieve a desired process objective, such as attaining the safe and efficient operation of machines and equipment utilized in the process. Process control systems have widespread application in the automation of industrial processes such as the processes used in chemical, petroleum, and manufacturing industries, for example.
Control of the process is often implemented using microprocessor-based controllers, computers or workstations which monitor the process by sending and receiving commands and data to hardware devices to control either a particular aspect of the process or the entire process as a whole. The specific process control functions that are implemented by software programs in these microprocessors, computers or workstations may be individually designed, modified or changed through programming while requiring no modifications to the hardware. For example, an engineer might cause a program to be written to have the controller read a fluid level from a level sensor in a tank, compare the tank level with a predetermined desired level, and then open or close a feed valve based on whether the read level was lower or higher than the predetermined, desired level. The parameters are easily changed by displaying a selected view of the process and then by modifying the program using the selected view. The engineer typically would change parameters by displaying and modifying an engineer's view of the process.
Many process control systems include local field devices such as valves, motors, regulators and the like which are responsive to specific control protocols, such as Profibus, Fieldbus, CAN and the like, to implement various control function routines. Accordingly, these controllers are responsive to certain standard control protocols to implement control functionality in the field. The use of such standard control signal protocols can reduce the time and effort of developing a control system because a designer can use the same types of control signals from all devices responsive to the control protocol.
In a conventional process control system, the local field devices are typically configured in the field, often by individually programming the local field devices using a hand-held field programmer. Individual programming of the field devices is time consuming and inefficient and often leads to incompatibilities between the device configuration and the configuration of other devices and controllers in the process control system since a global view of the system is more difficult to sustain when individual devices are programmed independently. Usage of individual programming devices is inconvenient since multiple different programming devices typically must be used to program respective different field devices.
Furthermore, local device failures, including temporary failures or local power disruptions, interrupt operations of the entire control system, sometimes causing extended downtime since each failing device must be reconfigured locally.
What is needed is a process control system that allows individual field devices to be configured without local, independent programming. What is also needed is a process control system allowing configuration of the global system from a location remote from the local field devices so that a compatible global configuration is achieved while allowing peripheral elements which are configured in a suitable global manner, to operate independently to achieve control functionality.
Configuration of the global system is based on parameters that describe the particular field devices that make up the system. However, the control protocols for communicating with the field devices may be insufficient to convey parameters that are sufficient to configure the system. For example, the system management specification of the Fieldbus protocol defines three states for a device including an INITIALIZED state, an UNINITIALIZED state, and a system management operational (SM OPERATIONAL) state. The three defined states are sufficient to describe the behavior of a device from the perspective of the system management, but are not adequate for describing a device from the perspective of either the fieldbus interface or software engineering tools for analyzing, controlling, or displaying the status of a device. This insufficiency is highly notable when configuration involves the operation of commissioning a device that is attached to the Fieldbus link in an UNINITIALIZED state.
What is further needed is a process control system that differentiates between Fieldbus device states to support automatic sensing of devices and online address assignment of devices.